The Boston Marathon bombers’ twisted sister left her boyfriend’s ex-girlfriend dumbstruck with terror when she allegedly threatened to kill the woman during a frightening phone call earlier this week, a friend said Tuesday.

“She was freaked out,” said a pal of Roslyn Betances, 23, who was allegedly told by her ex-boyfriend’s new squeeze, Ailina Tsarnaeva, that “I know people who can put a bomb on you.”

“She got into a panic attack and she wasn’t even talking,” the friend said. “She wasn’t responding at all.”

In addition to being speechless, the terrorized Betances’ face went flush, said the friend, who would only identify herself as Rachel.

“She was scared. I mean anybody would be, especially with the person knowing where you live here.”

She was freaked out… She got into a panic attack and she wasn’t even talking. She wasn’t responding at all.

- Roslyn Betances' friend 'Rachel'

Tsarnaeva’s boyfriend, George Briones, has three children with Betances, who lives in Harlem.

Monday’s explosive phone call is believed to be part of an ongoing custody battle between the couple — and into which Tsarnaeva has injected herself.

The terrorist sibling surrendered to police at the 30th Precinct Wednesday after the woman reported the threat. The sister of the marathon attackers was interviewed by the NYPD Bomb Squad, as well as the Joint Terrorism Task Force.

She admitted only to calling Betances and telling her, “Leave us alone” — but denied mentioning anything about a “bomb.”

Tsarnaeva allegedly claimed it was Betances who was the one who threatened her.

Tsarnaeva was charged with aggravated harassment, given a desk appearance ticket and released.

A law-enforcement source said Tsarnaeva’s “threat” was likely just hot air.

“She was very nice,” the source said. “She was crying for a little while while waiting to be interviewed, because she was frustrated being there.

“I don’t think she had any plans to blow up anything — hopefully not.”

But Betances’ friend said this isn’t the first time the sister of the Boston bombers has threatened her, the woman’s friend said.

Tsarnaeva, who lives in North Bergen, NJ, allegedly threatened “to f–k her up,” when Betances tried to serve Briones with legal papers this past June ordering him to appear in court, adding, “I already know where she lives.”